SENSITIVENESS. 57 



marked characteristics, and this tendency to grow, or 

 move towards the centre of the earth under the influ- 

 ence of gravitation, is known as "geotropism," the op- 

 posite tendency being called f< apogeotropism." Knight 

 was the first to show that downward tendencies of the 

 root were due to gravitation, and this he did by causing 

 seedlings to grow on a wheel kept in motion. The effect 

 of gravity was here overcome by the movement of the 

 wheel, and the rootlets, instead of growing downwards, 

 were now directed away from the centre of the wheel. 

 Darwin shows (1. c., p. 540) that it is the tip of the root 

 alone that is involved in this downward tendency, the 

 destruction of the tip putting a stop to the movement. 

 While the primary root or radicle under favorable cir- 

 cumstances penetrates the soil perpendicularly downwards, 

 the secondary ones bend obliquely, not perpendicularly, 

 downwards, the tertiary ramifications and their subdi- 

 visions being so little affected by geotropism that they 

 grow out freely in all directions. From this manner of 

 growth in the main root and its branches respectively, it 

 is evident how the whole mass of soil within their reach 

 becomes, under favorable conditions, a happy hunting 

 ground for the roots. Moreover, it has been shown that 

 where the primary radicle, the origin of the " tap" root, 

 has been destroyed as it often must be in nature, by 

 insects or other means the secondary roots, instead of 

 retaining their oblique direction, assume that previously 

 taken by the injured root and pass downwards. 



The Action of Lteht and Heat on Roots, The direct 

 action of light upon ordinary roots is, of course, usually 

 of a negative character. The form and direction of 

 growth in the root may, however, be affected by differ- 

 ences of temperature, experienced now on one side, now 

 on another. Darwin has shown that the movements of 

 roots, due to irritation or contact, are checked by too 



